It is known to use detectors that have a secondary-electron emitter to detect radiation of charged particles or photons. The emitter can emit secondary electrons upon being irradiated. The emitted electrons are collected to produce a signal to indicate the presence or a characteristic, such as energy, of the radiation.
Typically, not every radiation quanta incident on an electron emitter causes an emission. Thus, radiation detectors have less than 100% detection efficiency. It is desirable that a radiation detector has high detection efficiency.
For example, PCT application publication WO 03/075299, published on Sep. 12, 2003, entitled “Device for detecting charged particles and photons,” to Bernd Fischer and Marian Cholewa (“Fischer”); and a report authored by M. Cholewa and E. koshchiy, “Thin Diamond film as highly efficient detector for changed particles,” Gesellschaft für Schwerionenforschung (GSI) Scientific Report 2003, (2004), p. 156, (“Cholewa”), each of which is incorporated herein by reference, disclose a detector consisting of a thin diamond layer on a supporting Si substrate and an electron multiplier unit. The diamond film is boron-doped and has high secondary electron emission yield. The detector is said to have almost 100% efficiency for detecting radiation of ions. However, detection efficiency for light radiations such as X-rays has not been reported and is expected to be lower. Further, boron-doped diamond film can be expensive and the production cost for radiation detectors having boron-doped diamond film as the electron emitter can be high. Thus, alternative or cheaper electron emitters with similar or even higher detection efficiencies are still desirable.
Several other techniques can be used to improve detection efficiency. For example, increasing the energy of incident radiation or subjecting the emitter to a high biasing voltage can increase emission efficiency. However, these techniques have limited applications and have limited effects. For instance, the energy of radiation to be detected may not be controllable; the biasing voltage cannot be increased without limit. It is desirable that high detection efficiency can be achieved over a wide range of radiation energy, biasing voltage, and types of radiations.
It has been reported that MgO coated on the tips of vertically-grown multiwalled carbon nanotubes (MWCNTs) can produce high secondary electron emissions by applying a high bias voltage of 800V. See for example, Won Seok Kim, Applied Physics Letters, (2002), vol. 81, pp. 1098-1100 (“Kim”), which is incorporated herein by reference. However, in this approach, good quality of MgO coating can only be produced by electron-beam evaporation, which leads to certain drawbacks such as limited coating area and high cost of equipment and production. Further, emitters that can exhibit even higher emission yields at low bias are still desirable.
Thus, there is a need for radiation detectors with alternative or improved electron emitters and have high detection efficiencies. Further, there is a need for radiation detectors that can be manufactured at reduced costs.